1. Field of the Invention:
The present invention relates to a vertical type construction transistor.
2. Description of the Related Art:
With the development of an epitaxial growth method for a compound semiconductor layer, there has arisen a demand for a further improved vertical type construction transistor including a semiconductor heterojunction. As a heterojunction bipolar transistor (HBT), one kind of vertical type construction transistor, operates with high current density, it should be made so that the generated heat can be dissipated to the outside.
FIG. 4A is a plan view showing a structure for conventional vertical type construction transistors 23. FIG. 4B is a cross-sectional view taken along the line of 4B-4B of FIG. 4A. Each vertical type transistor 23 has a collector layer 24 formed on a semiconductor substrate 20 including a sub-collector layer, a base layer 21 formed on the collector layer 24, and emitter layer 22 formed on the base layer 21. A finger-shaped emitter electrode 1 and a finger-shaped base electrode 2 are formed on the emitter layer 22 and the base layer 21, respectively. A collector electrode 3 is formed on the semiconductor substrate 20. The emitter electrode 1 and the base electrode 2 are extended from the opposite side to each other so as to be in parallel with each other. The emitter electrode 1 and the base electrode 2 are connected to an emitter lead electrode 11 and a base lead electrode 12, respectively. The collector electrode 3 is connected to a collector lead electrode 13. The emitter lead electrode 11 is further connected to emitter electrodes of the adjacent vertical type construction transistors 23, and the collector lead electrode 13 is further connected to a collector electrode 3 of the adjacent vertical type construction transistor 23.
According to the vertical type construction transistor 23, remarkably high heat is generated at a junction portion between the collector layer 24 and the base layer 21. A via hole is provided so as to extend through the emitter lead electrode 11 from the reverse face side of the semiconductor substrate 20, and then a metal excellent in thermal conductivity is buried into the via hole 4, so that the generated heat can be dissipated to the outside from the reverse face side of the semiconductor substrate 20 through the emitter electrode 1 and the emitter lead electrode 11. However, according to such a structure, the generated heat cannot be effectively dissipated to the outside, so that the vertical type construction transistor 23 inevitably has high thermal resistance. Moreover, since the emitter lead electrode 11 is formed into a narrow-strip shape, the emitter inductance becomes large during the operation in a high frequency band, thereby lowering the gain. As many problems such as those are caused, conventionally the vertical type construction transistor could not be put to practical use in the high frequency band.
FIG. 5 shows a cross-sectional view of another conventional vertical type construction transistors 25. Each vertical type transistor 25 has a collector layer 24 formed on a semiconductor substrate 20 including a sub-collector layer, a base layer 21 formed on the collector layer 24, and emitter layer 22 formed on the base layer 21. A finger-shaped emitter electrode 1 and a finger-shaped base electrode 2 are formed on the emitter layer 22 and the base layer 21, respectively. A collector electrode 3 is formed on the semiconductor substrate 20. The collector electrodes 3 of the adjacent vertical type construction transistors 23 are each connected to a common collector lead electrode 13, which is arranged in the air so as to cross over the emitter electrodes 1 and the base electrodes 2 by using an air-bridge technique.
Due to such a structure for the collector lead electrode 13, parasitic capacitance C.sub.BC between base/collector is increased, so that the characteristics of the vertical type construction transistor 23 are deteriorated in the high frequency band. In addition, it is difficult to form the collector lead electrode 13 in an arched shape so as to cross over the emitter electrodes 1, which results in a low production yield and a high production cost. Moreover, the emitter electrode 1 should be made thin so as not to contact with the collector lead electrode 13, and therefore operation errors will be likely to occur. Furthermore, even when the heat of the junction portion between the emitter layer 22 and the base layer 21 is dissipated through the emitter electrode 1, the heat cannot be dissipated to the outside but rather confined in a space 5 defined by the collector lead electrode 13 formed above the emitter electrode 1.
Accordingly, the present invention has been developed with a view to substantially eliminating the above discussed drawbacks inherent in the conventional vertical type construction transistor. The objective of the present invention is to provide an improved vertical type construction transistor, especially a vertical type construction transistor to be operated at high speed in the high frequency band, by forming lead electrodes which can effectively dissipate the heat of the junction portion, and provide low inductance during high frequency operation. Therefore, the vertical type construction transistor of the present invention can be put to practical use as a power amplification use in the microwave band.